What does the torn sheet of paper icon in outliner mean?
It means fiie missing or broken link. In this case it might mean that the linked file that contains the object (or collection) cannot be found in the disk. Did you move, rename or delete any files?.
To find the error, switch the outliner display the blender file mode and look for missing libraries.

Or display API data, navigate to the missing collection and find what is it that is linked to. You can re-link it to the original file by manually typing the correct path.

When you get an error like this,you can also check the terminal output. It should report something like: